The present invention relates to a cathode ray tube having an oxide cathode and a process for preparing the same, particularly an electron emissive material layer in the cathode and a process for preparing the same.
In FIG. 14, a cross sectional view of a conventional oxide cathode is illustrated as a model which is described, for instance, in JP-A-8-77914. In the figure, 101 is a metal substrate which contains nickel as a major component and a reducer of, for instance, silicon and magnesium. The metal substrate 101 is a circular plate constituting the bottom of a long-cavity cylindrical sleeve 102. And 103 is an electron emissive material layer mainly comprising a needle-like particle 105 of oxides of alkaline earth metal such as barium, strontium and calcium, which adheres to the metal substrate 101. Also, 104 is a filament which is heated in order that thermoelectrons are emitted from the electron emissive material placed in the sleeve 102. The oxide cathode is placed in an evacuated cathode ray tube (no figure). In FIG. 14, the size of the needle-like particle 105 is enlarged about 10 times larger than the size (diameter and thickness) of the electron emissive material layer 103. Therefore, among the needle-like particles 105, except the needle-like particles 105 contacted to the metal substrate 101, the real width shows about one tenth of the total width from the surface.
A process for preparing the oxide cathode of the cathode ray tube is as follows:
At first, particles of the alkaline earth metal carbonate are dispersed in an organic solvent to prepare a dispersion solution (paste) having suitable viscosity for spraying. By repeating a process of spraying the paste onto the metal substrate 101 and drying the same, the pre-determined thickness, for instance, 40 to 100 xcexcm is obtained. The oxide cathode is placed in the cathode ray tube, and by forming a vacuum inside the cathode ray tube, the cathode ray tube is heated from outside or by the filament 104. The organic solvent is decomposed and evaporated up to about 600xc2x0 C., and by heating up the carbonate salt to about 900 to 1000xc2x0 C., the carbonate salt is decomposed to give an oxide and the electron emissive material layer 103 is formed which emits electron.
The particles of the alkaline earth metal carbonate which form the electron emissive material is usually shaped like a needle and one of them is shown in a large scale in FIG. 15. As is shown in FIG. 15, the longest length of the particles of the alkaline earth metal carbonate 105 is defined as L xcexcm and the longest axis vertical to the direction is defined as D xcexcm, while the same definition is applied to the particles having nearly spherical shape in the followings. Usually, the particles having an average length L of about 4 to 15 xcexcm and an average diameter D of about 0.4 to 1.5 xcexcm are used as the particles of the carbonate. Though the oxide after decomposition process is slightly shrunk, the shape is almost kept. Due to the shape and the size of the particles and application by spraying, suitable voids are made to achieve high electron mission and a long duration. On the other hand, JP-A-8-77914, discloses an art in which a variation in the thickness of the electron emissive material layer is reduced and a long duration is achieved by partly using spherical or branched particles. JP-A-59-191226 discloses an art in which a paste of a carbonate salt is applied by printing.
The above preparation process of spraying may cause large unevenness of the surface of the electron emissive material layer as shown in FIG. 14, and therefore, electron beam is distributed irregularly along the uneven surface. The reason is, for example, that in case an electric field on the surface of the electron emissive material layer is not large, the electric field converges on the top of the convex and the electron emission of the convex part becomes larger than that of the concave part. A distribution of electron beam is preferably Gaussian distribution. When the distribution is irregular, there is a problem that a pitch of a shadow mask is interfered and moire easily occured.
In case unevenness of the surface of the electron emission layer was large, there was a problem that direction of electron emission tended to be easily expanded, and therefore, the beam tended to be expanded and resolution became lowered. On the other hand, in order to reduce the unevenness of the surface of the electron emissive material layer, there can be considered a process in which a paste containing the alkaline earth metal carbonate which forms the electron emissive material layer is applied on the metal substrate by printing. In the process, however, there was a problem that no suitable voids were made on the electron emissive material layer and an amount of the electron emission was smaller than that obtained by the process of spraying.
The present invention has been made in order to solve the above problems, and there is provided a process in which surface unevenness of an electron emissive material layer is reduced and a suitable voids are formed to obtain a cathode ray tube having a little moire and high resolution. In brief, the electron emissive material layer is constituted by using two particle groups having a different shape, and by applying a process in which the shape and the ratio of the particles is specified, an excellent cathode ray tube is obtained.
The first oxide cathode of the cathode ray tube of the present invention comprises an electron emissive material layer having an alkaline earth metal oxide on a metal substrate containing nickel as a major component, wherein the alkaline earth metal oxide comprises a mixture of needle-like particles of the first group and bulk particles of the second group which is different from the particles of the first group. An average length of the particles of the second group is at most 60% of that of the first group particles, an average diameter of the particles of the second group is at least 1.5 times larger than that of the first group particles and a ratio of the particles of the first group in the alkaline earth metal oxide constituting the electron emissive material layer is 50 to 95% based on the atomic ratio of the alkaline earth metal oxide.
The second oxide cathode of the cathode ray tube of the present invention is that in the oxide cathode of the first cathode ray tube, the particles of the second group are spherical particles having an average diameter of at most 7 xcexcm.
The third oxide cathode of the cathode ray tube of the present invention is that in the oxide cathode of the first or second cathode ray tube, the particles of the second group comprises an oxide of at least barium and strontium, and the total amount of barium in the particles of the second group is at most 30% based on the atomic ratio of the alkaline earth metal oxide of the particles of the second group.
The fourth oxide cathode of the cathode ray tube of the present invention is that in the oxide cathode of the first, second or third cathode ray tube, the base on which an electron emissive material layer of a metal substrate is formed is a nearly circular shape having a diameter of r1 (mm) and the planar shape of the electron emissive material layer is a nearly circular shape having a diameter of r2 (mm), and the following equation is satisfied.
r2xe2x89xa6r1xe2x88x920.1
The fifth oxide cathode of the cathode ray tube of the present invention further has a layer containing, as a main component, tungsten or molybdenum between the metal substrate and the electron emissive material layer of the oxide cathode of the first, second, third or forth cathode ray tube.
The first process for preparing the oxide cathode of the cathode ray tube of the present invention comprises a process for applying, by printing, the paste for printing containing particles of the alkaline earth metal carbonate forming the electron emissive material on the metal substrate which contains nickel as a main component and constitutes the oxide cathode, a drying process in which the paste for printing applied in the above process is fixed on the metal substrate, and a process for heating during forming a vacuum to oxidize the alkaline earth metal carbonate to an oxide as the electron emissive material after the oxide cathode is incorporated in the cathode ray tube, wherein as the alkaline earth metal carbonate, there is used a mixture of needle-like particles of the first group and bulk particles of the second group which is different from the particles of the first group, an average length of the particles of the second group is at most 60% of that of the first group particle, an average diameter of the particles of the second group particle is at least 1.5 times larger than that of the first group particle, and a ratio of the particles of the first group in the alkaline earth metal oxide constituting the electron emissive material layer is 50 to 95% based on the atomic ratio of the alkaline earth metal oxide.
The second process for preparing the oxide cathode of the cathode ray tube of the present invention comprises a process for applying, by printing, the paste for printing containing particles of the alkaline earth metal carbonate forming the electron emissive material and particles for producing voids having an average diameter of 1 to 20 xcexcm on the metal substrate which contains nickel as a main component and constitutes the oxide cathode, a drying process in which the paste for printing applied in the above process is fixed on the metal substrate, and a process for heating during forming a vacuum to oxidize the alkaline earth metal carbonate to an oxide as the electron emissive material after the oxide cathode is incorporated in the cathode ray tube, and in which the above particles for producing voids are removed while heating.
The third process for preparing the oxide cathode of the cathode ray tube of the present invention is that in the second process for preparing the cathode ray tube, a volume ratio of the particles for producing voids against the alkaline earth metal carbonate is 5 to 30%.
The forth process for preparing the oxide cathode of the cathode ray tube of the present invention is that in the third process for preparing the cathode ray tube, the particles for producing voids is acrylic resin powder.
The fifth process for preparing the oxide cathode of the cathode ray tube of the present invention is that in the second process for preparing the cathode ray tube, as the alkaline earth metal carbonate in the paste for printing, there is used a mixture of needle-like particles of the first group and bulk particles of the second group which is different from the particles of the first group, an average length of the particles of the second group is at most 60% of that of the first group, an average diameter of the particles of the second group is at least 1.5 times larger than that of the first group, and a ratio of the particles of the first group in the alkaline earth metal oxide constituting the electron emissive material layer is 50 to 95% based on the atomic ratio of the alkaline earth metal oxide.
The sixth process for preparing the oxide cathode of the cathode ray tube of the present invention is that in the first, second, third, forth or fifth process for preparing the cathode ray tube, the process for applying the paste by printing is carried out by using a screen printing.
The seventh process for preparing the oxide cathode of the cathode ray tube of the present invention is that in the sixth process for preparing the cathode ray tube, the paste for printing contains at least one of a nitrocellulose solution and an ethylcelluloce solution, terpineol and a dispersion agent, the paste has a viscosity of 2,000 to 10,000 cP, a mesh of No. 120 to 500 is used during the process of applying the paste for printing and the thickness of the paste for printing after the drying process is 40 to 150 xcexcm.
The eighth process for preparing the oxide cathode of the cathode ray tube of the present invention is that in the sixth or seventh process for preparing the cathode ray tube, the surface of the metal substrate forming the electron emissive material layer is a nearly circular shape having a diameter of r1 (mm) and the shape of an opening part of a mask for screen printing is a nearly circular shape having a diameter of r2 (mm), and satisfying
r2xe2x89xa7r1xe2x88x920.1.
The ninth process for preparing the oxide cathode of the cathode ray tube of the present invention is that in the first, second, third, forth, fifth, sixth, seventh or eighth process for preparing the cathode ray tube, the surface of the metal substrate is concave, on which the electron emissive material layer is formed.
The tenth process for preparing the oxide cathode of the cathode ray tube of the present invention is that in the first, second, third, forth, fifth, sixth, seventh, eighth or ninth process for preparing the cathode ray tube, shape of the paste for printing after drying or shape of the electron emissive material layer is convex toward a direction of electron extraction, at least on the part from which the electron is extracted.
The eleventh process for preparing the oxide cathode of the cathode ray tube of the present invention is that in the tenth process for preparing the cathode ray tube, the surface of the metal substrate is convex on which the electron emissive material layer is formed.